In 1990 made his television debut with the series Twin Peaks. The series was created by David Lynch and Mark Frost and focused on the investigation of the murder of a prom queen. Fienberg served as a production manager and producer for the first season. Fienberg and the production staff were nominated for an Emmy Award for Outstanding Drama Series at the 42nd Primetime Emmy Awards. He returned as a production manager and supervising producer for the second and final season in 1991. He served as unit production manager for the follow-up feature Twin Peaks: Fire Walk With Me in 1992. Also in 1992 Fienberg was a production manager for the pilot of Lynch and Frost's short-lived next series, On The Air.

In 2001 Fienberg began a long association with the HBO cable network. He served as a co-executive producer for the first season of the comedy The Mind of the Married Man in 2001. The series was created by and starred Mike Binder. Fienberg left the crew after the first season. Fienberg was also a co-executive producer for the first season of HBO depression era drama Carnivàle in 2003. The series was created by Daniel Knauf. The show focused on a traveling carnival and their supernatural ties to a Christian minister. Fienberg again left the crew after the first season.

In 2007 Fienberg became an executive producer and director for HBO surf noir John From Cincinnati. The series was created by Milch and focuses on the arrival of a messiah like figure in a surfing community. Fienberg directed the episode "His Visit: Day Two". The series was canceled after one season.

In 2009 Fienberg became unit production manager and executive producer for the second season of HBO supernatural drama True Blood. The series was created by Alan Ball and focuses on a small Louisiana town after Vampires reveal their existence to the world. He will return as an executive producer for the third season in 2010. In September 2013, HBO officially announced that True Blood had been renewed for a seventh and final season that will premiere in Summer 2014.